Unshielded, twisted-pair cable of the type conventionally used for modular telephone lines is finding increased use for digital applications and particularly for local area network (LAN) systems. In addition, the recent introduction of the "10baseT" system for Ethernet networks is expected to a profound impact and may result in a substantial increase in the use of unshielded twisted-pair cable for data applications.
The use of unshielded twisted-pair cable instead of shielded cables and coaxial cables provides today's LAN user with substantial cost savings and flexibility. Not only is the twisted-pair cable substantially less expensive than the more traditional data cables, but in many instances existing telephone lines already include spare twisted pairs, so that required cable runs are already in place. This means that LANs can be installed and reconfigured as easily as telephone systems are reconfigured.
With the use of appropriate adapters, nearly all of the major LAN systems can operated on existing telephone cables. However, the absence of shielding in the cable presents an electromagnetic interference problem. High frequency components of the signals being transmitted in the cable can cause radiation into the environment, which can interfere with or otherwise affect other electronic or broadcast equipment. On the other hand, radiation from other equipment can produce interference with the unshielded cable, reducing the reliability of transmission and, in some instances, making effective transmission impossible. The FCC regulations on computer equipment define the limits of such emissions and interference.
Broadly, it is an object of the present invention to minimize or eliminate electromagnetic interference (EMI) in data transmission applications utilizing twisted-pair cable.
It is a specific object of the invention to provide a communication filter which provides EMI protection in LAN applications of twisted-pair cable.
It is also an object of the present invention to provide a communication filter providing EMI protection for twisted-pair cable applications, which filter is convenient and reliable in use, yet relatively inexpensive and simple in construction.
In accordance with a preferred embodiment demonstrating objects and features of the present invention, the data signal is transmitted differentially on two conductors constituting the twisted-pair cable, and a data filter is provided which includes an in-line transformer and a shunt transformer. The in-line transformer has a winding (i.e. the primary or secondary) in series with each line of the twisted pair. These windings are arranged so that they are wound in the same direction. Similarly, the shunt transformer includes a winding which shunts each wire of the twisted pair to ground through a series capacitor. These shunt windings are wound in opposite directions to each other. Owing to the manner in which the two transformers are wound, a data signal, which is applied differentially to the twisted pair will not be impeded by the in-line transformer, but will be impeded by the shunt transformer. A noise signal, such EMI, be introduced with the same polarity on both conductors of the twisted pair and will therefore experience the in-line transformer as a high impedance, whereas the shunt transformer will be low impedance, and the capacitor in series therewith will also be low impedance. Accordingly, noise signals are shunted to ground.